1. Field of the Invention
The present invention relates generally to a support pin system which is a non-welded mechanical system and, more particularly, to a novel support pin system which may include a novel locking nut and/or a novel locking nut retainer, for utilization especially in connection with nuclear reactor control rod guide tubes to fixedly secure the control rod guide tubes by means of their annular flanged portions to the upper surface of the upper core plate of the reactor by means of such a nonwelded mechanical system.
2. Description of the Prior Art
Support pin systems for fastening a first structural member to a second structural member by means of a support pin and nut are utilized in applications requiring a non-welded mechanical system, such as the support pin system for fastening a nuclear reactor control rod guide tube flange to a nuclear reactor upper core plate whereby control rod guide tubes are secured in the upper reactor internals between the upper core plate and an upper support plate. Within a nuclear reactor, as is well known, the upper boundary of the reactor core is defined by means of an upper core plate. The upper ends of the nuclear reactor core fuel assemblies are connected to the under surface of the upper core plate, as shown, for example, in U.S. Pat. No. 4,173,153 to Obermeyer et al, the disclosure of which is herein incorporated by reference. Each fuel assembly has fuel rods and has defined therein numerous locations within which nuclear reactor control rods are capable of being disposed for regulating the power output of the fuel assemblies and the reactor core. Protection for the control rods is provided, with respect to the crosscurrents of the flowing nuclear reactor core coolant, through means of guide tubes which are interposed between and fixedly connected to, the upper surface of the upper core plate and an upper support plate which is disposed above and vertically spaced from the upper core plate.
The guide tubes are provided with annular flanges at the lower ends thereof and guide tube support pins fixedly interconnect the guide tube flanges to the upper core plate. The vertically disposed guide tube support pins have split-leaf lower portions which are disposed within and frictionally engage suitable bores within the upper core plate, and upper bolt portions which pass through through-bores provided in the guide tube flange and threadedly engage suitable, internally threaded nuts. Counter-bored portions of the guide tube flange are generally retained between an annular shoulder portion of the support pin shank and the mated nut. In order to prevent the retrograde rotation of the nut relative to the support pin whereby the nut may possibly become disengaged from the threaded portion of the guide tube support pin, a dowel pin is conventionally passed through the nut and the exposed ends welded to support pin locking tabs.
While the aforenoted conventional locking system is entirely satisfactory when the various components of the nuclear reactor system are initially installed prior to commencement of plant operations, once plant operations have commenced, and the support pins require repair or replacement, for example, due to failures as a result of stress corrosion cracking problems, repair or replacement of the aforenoted welded-type dowel pin locking system cannot be simply effectuated in view of the fact that such welding operations must now be performed remotely in order to protect maintenance personnel from radiation exposure within the irradiated environment, and the operations must also be performed in a spacially restricted or confined underwater environment. Further, welding to secure the nut to the support pin, whether performed directly or indirectly by means of the aforementioned dowel pin and tab system, is disadvantageous. The welding heat adversely affects the pre-load of the nut torqued onto the support pin, which preload provides resistance to flow-induced vibration. Further, structural material adjacent to the welded area may be adversely affected, the welding heat tending to promote subsequent corrosion and stress cracking of the affected materials.
A dual crimp locking system has been disclosed in the copending, commonly-assigned, U.S. patent application Ser. No. 576,645 by J. T. Land et al, filed on Feb. 3rd, 1984, the disclosure of which is herein incorporated by reference. A support pin having a split-leaf base section, an externally threaded, upper bolt portion and a top end portion provided with vertical grooves is disclosed in which the grooved top end protrudes through a securing nut threaded onto the upper bolt portion. The nut is provided with vertical splines and the locking system is secured by means of a stepped tubular cap which is crimped into place around both the top end portion of the support pin and the securing nut whereby retrograde rotation between the support pin and nut is positively prevented. Such a fastening arrangement, however, depends for its efficacy on the structural integrity of the crimped cap, i.e., the stepped tubular cap, as well as that of the support pin. The structural configuration of these support pins, however, subjects them to both high shank preload stresses and to high bending loads and stress corrosion cracking and shearing, especially of the shank portion above the annular shoulder, has been observed, especially for the heretofor preferred Inconel-750 material. The affected nuclear reactor guide tube is thus left without a properly secured support.
Moreover, the longevity of the nuclear reactor as a whole may be seriously compromised by dislodged parts which may be propelled through the system by the swiftly flowing nuclear reactor core coolant and damage other power plant components, such as the steam generator. When the shanks of conventional support pins shear under the combined influence of the high preload and high bending stresses, the upper bolt portion of the support pin, along with the attached nut and/or crimped cap, may subsequently dislodge under the influence of the coolant flow. The nut, crimped cap or dowel pin, etc., may also disengage. The split leaf base section and/or one or more of the leaves may disengage, all with deleterious result. Further, all presently-known guide tube support pin and locking systems have one or more of the above recited disadvantages.
Accordingly, it is an object of the present invention to provide a new and improved support pin system, which is a non-welded mechanical system, and is useful especially in connection with nuclear reactor control rod guide tubes.
It is another object of the present invention to provide a locking nut retainer for positively retaining a nut in position around an elongate threaded element when the retainer is positioned within and accommodated by a structural member having the elongate threaded element disposed therein, such as for positively retaining a nut in position around a support pin for fastening a nuclear reactor control rod guide tube flange to a nuclear reactor upper core plate.
It is yet another object of the present invention to provide a locking nut for positively preventing relative rotation of the nut when threadedly engaged to an elongate threaded element, which locking nut may be used in the above support pin system and/or may be used in conjunction with the above locking nut retainer.